Optical systems, such as fiber optic communication systems, may include optical transmitters to transmit optical signals and optical receivers to receive the optical signals from the optical transmitters using multiple optical channels (e.g., with different carriers) on single fibers. Optical receivers may detect incoming optical signals on these single fibers by using direct detection of the optical signals. A disadvantage of direct detection is the inability to distinguish an optical channel's frequency due to the optically broadband nature of common photo-detectors. As such, if more than one optical channel is incident on a direct detection receiver, more than one optical channel may be converted to baseband by the photo-detector (e.g., demodulated electrical output) and each converted optical channel may interfere with each other and degrade the quality of the information carried in the optical channels. This interference may be referred to as crosstalk.
In the case of direct detection, if more than one optical channel is incident on a receive port, a fixed optical frequency discriminator (e.g. optical filter) may be used in the receiver to reduce the crosstalk issue described above. Alternatively, a tunable optical filter may be employed in place of the fixed optical filter. The tunable filter may be able to provide a flexible optical frequency selection at the receive port, but may be expensive and complex compared to alternative optical frequency selective architectures that select the optical frequency prior to the receive port (e.g., optical add/drop multiplexer (OADM) or reconfigurable OADM (ROADM) architectures).
ROADMs provide a means for building optical networks that automate add/drop optical frequency assignments. The optical frequency to optical port assignment on the add/drop can be fixed or reconfigurable. For the later, colorless ROADM functionality, in which multiple channels at multiple optical frequencies may be broadcast to multiple receivers, may be used to allow flexible reconfiguration of optical connections as network requirements change. The broadcasting of multiple channels to an optical port may force a receiver to be able to discriminate an incident channel (or channels) of interest from other incident channels, which may lead to complications, such as those described above in the case of direct detection receivers.
One way to provide colorless ROADM functionality is to use coherent detection with a local oscillator (LO) that may serve as an optical frequency discriminator (e.g., substituted for an optical filter). For example, optical coherent heterodyne or homodyne detection can be used. In heterodyne detection, an optical signal (or channel) of interest (at some frequency) is non-linearly mixed in a photodiode with a reference LO optical frequency. The reference LO optical frequency may be set at an optical frequency close to the optical signal. A resulting current from the photodiode may carry the original optical signal information (e.g., amplitude, phase, and frequency modulation) of the original signal at the optical frequency, but may be centered at a difference frequency (e.g., a difference between signal optical frequency and LO optical frequency). This resulting current electrical representation of the optical signal can be electrically filtered.
If the LO optical frequency is offset relative to the optical signal by less than a particular amount (e.g., a symbol rate or analog bandwidth of the optical signal) of the optical signal, typically referred to as “intradyne” detection, simple low pass electrical filtering may be sufficient to reject unwanted channels. The electrical filtering can be accomplished via: an addition of at least one dedicated electrical filter (e.g., low-pass or bandpass) after the photodiode; the inherent bandwidth of the electrical functions filtering the unwanted channels; or a combination thereof. In the case of multiple optical signals (or channels) present at the photodiode, the electrical filtering can be used to reject unwanted channels from subsequent signal processing to provide frequency discrimination. However, while a coherent receiver may utilize the LO as a frequency discriminator, the coherent receiver may not be able to determine the quantity and/or type of channels that may be incident on the coherent receiver.
The quantity and/or type of channels incident on a receiver may be provided to the receiver (or a control module of the receiver) by a network control plane (e.g., hardware or a combination of hardware and software that may control an optical system and may have set the quantity of channels initially). However, if a channel is provided from a source not controlled by the network control plane (e.g., an alien optical frequency channel from an outside or third party source), then the network control plane may not have information about this channel. Without information about the channel, the network control plane may not be able to provide the receiver with the number of channels that may be incident on the receiver. In this case, these channels (e.g. alien optical frequency channels), including the number of channels incident to a receiver, may be unknown to the network control plane and therefore unknown to the receiver.
If the quantity of channels incident is unknown by a network control plane, the quantity of channels incident on a receiver may be detected by the LO. The LO may sweep across possible optical channel frequencies looking for the presence of signal channels at each optical channel frequency (e.g. by monitoring the photocurrent from at least one photodiode in the receiver) to find the quantity of channels incident on the receiver. Using the LO in this manner, however, may be service impairing or disrupting due to the nature of the LO sweep, as described above.